1. Field of the Invention
The present invention relates to a position measuring device.
2. Description of the Related Art
Such position measuring devices are employed in particular in connection with processing machinery for measuring the relative position of a tool in relation to a workpiece to be processed, with coordinate-measuring machinery for determining the position and dimensions of test objects, and lately increasingly in the semiconductor industry as well, for example in connection with wafer steppers. In this case the position measuring device is an angle or linear measuring device, wherein the scale is directly installed on the drive unit (rotatory motor or linear motor), or the scale is installed on a component driven by the drive unit.
The scale of such a position measuring device known, for example, from DE 92 09 801 U1, has a periodic scale graduation for generating counting signals, as well as reference markings for generating reference marking signals. An absolute relation of the position measurement will be produced for the position of the reference marking via this reference marking signal in that a counter is placed at a predetermined counter setting.
It is often necessary at the start of a position measurement following a break in the operation because of loss of energy supply, as well as for correcting and checking the counter setting, to approach the reference marking starting from an arbitrary position. To this end, area markings have been applied to the scale, by which it is possible to differentiate whether the scanning device is located on one or the other side of the reference markings. This area marking is embodied on one side of the reference markings in the form of a continuous opaque strip. On the other side of the reference markings the area marking is embodied as a transparent area. A common photo receiver is arranged in the scanning device for scanning both area markings. The decision as to whether the scanning device is located on one or the other side of the reference markings is made on the basis of a measuring signal from this photo receiver.
It has been shown that because of the signal levels of this measuring signal this differentiation is problematic, since the mutual distance between the levels is very short. This measuring signal can also easily be affected by partial soiling or contamination of the scale, so that a wrong position of the scanning device is detected.